In the first stanza, the poet describes a rough sea during the day time. The waves are rolling to the beach all day making a sound. So, in its rumbling, tumbling and rolling, it is like a dog. In the second stanza, the poet creates a picture of a very rough sea, at night in storm. The rise of the tide in the evenings is like the bounding or leaping of a dog sniffing and snuffing on its prey.

In the last stanza, the poet describes about a calm and quiet sea during May or June. Here, the poet creates a very fanciful image by transferring the ferocious, mischievous dog to a seemingly innocent, sleeping dog.
Further, both the sea and the dog reflect nature in its range and variety. The poet says the sea could be ferocious as well as gentle as a dog could be.